Apparatus for loading cavity resonators of tunable velocity modulation tubes

ABSTRACT

A tunable high frequency velocity modulation beam tube has a plurality of floating resonators arranged along a beam path intermediate an input circuit and an output circuit for velocity modulating the beam to provide bunching of the beam at the output circuit. The cavity resonators are tunable over a relatively wide band for tuning the electronic bandwidth of the tube over a relative wide tunable band. A plurality of floating resonators are each loaded by a series resonant loading circuit with the Q&#39;&#39;s and frequencies adjusted to provide a certain flat electronic bandwidth. The loading circuits are tuned and loaded such that the loss introduced into the floating resonators is frequency dependent over the tunable band of the tube in order to maintain the given electronic bandwidth over the tunable band of the tube.

United States Patent Levin 111 3,725,721 1 Apr. 3, 1973 Primary Examinerl-lerman Karl Saalbach Assistant ExaminerSaxfield Chatmon, Jr. Attorney-Stanley Z. Cole [75] Inventor: Martin E. Levin, San Mateo, Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif. [57] ABSTRACT Filedl y 17, 1971 A tunable high frequency velocity modulation beam [21] APPL No: 144,411 tube has a plurality of floating resonators arranged along a beam path intermediate an input circuit and an output circuit for velocity modulating the beam to gfi3 provide bunching of the beam at the output circuit. I}. J The cavity resonators are tunable over a relatively [58] Field of Search ..3l5/3.5, 5.43, 5.46, 5.39 wide band for tuning the electronic bandwidth of the tube over a relative wide tunable band. A plurality of [56] References Cited floating resonators are each loaded by a series reso- UNITED STATES PATENTS nant loading circuit with the Qs,and frequencies ad- 2 934 672 4 1960 P u k l 6 justed to provide a certain flat electronic bandwidth. I 0 ac eta The loading circuits are tuned and loaded such that 3,360,679 12/1967 Rubert ....3l5/3.5 h 3,365,607 lH968 Ruetz ct alum HMS/35 the loss introduced into t e floating resonators is 3,028,519 4/1962 Jepsen etal..... ....315/3.5 frequency dependent Over the tunable band of the 3,195,007 7/1965 Watson et al ..3l5/5.43 tube in order to maintain the given electronic band- 3,l04,340 9/1963 Crapuchettes ..3l5/5.39 width over the tunable band of the tube. 3,249,794 5/1966 Staprans et al. ..3l5/5.43 3,248,593 4/1966 Mihran et al. ..3l5/5.46 8 Claims, 12 Drawing Figures DRIVER CAVITIES 4 2 3 |,4 |4|| |,4|2 U14 l/ I L r r 5 z I l A l ri' I l 9 b 7/ b R FLOATING RE |N RESONATORS OUT PATENTEUAPR3 I973 725,721

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INVENTOR. MARTIN E. LEVIN BY ATTORNEY.

APPARATUS FOR LOADING CAVITY RESONATORS OF TUNABLE VELOCITY MODULATION TUBES DESCRIPTION OF THE PRIOR ART Heretofore, multi-cavity staggered tuned klystron amplifier tubes have been employed as the final transmitter tube for TV broadcasting. Such tubes are generally designed to be tunable over a relatively wide band such as from 575 MHz to 704 MHz and to provide an electronic bandwidth of approximately 8 MHz for each of the various channels within the tunable band. Such tubes are required to produce approximately 10 KW of average output power with a gain within 1 dB over the 8 MHz of electronic bandwidth.

In order for these prior art tubes to achieve a uniform gain over the electronic bandwidth, one of the floating resonators of the klystron driver section is usually tuned to the low band edge of the electronic bandwidth and the Q thereof loaded to approximately 200 at the low frequency end. Another one of the floating resonators is typically tuned near the high frequency end of the electronic bandwidth and its Q adjusted at the low frequency end of the range for approximately 200.

The klystron cavities typically have Qs substantially higher than those desired and, therefore, the cavities are loaded with a lossy material to reduce their 0. Heretofore, the loss has been introduced by coating the inside walls of the cavity with a lossy material, such as Kanthal, or by coupling the cavity to an external resistive load.

The problem with the prior art tubes has been that when the Qs of the various cavities are adjusted for a given electronic bandwidth at the low end of the tunable The problem with the prior art tubes has been that when the Qs of the various cavities are adjusted for a given electronic bandwidth at the low end of the tunable band and then the tube is tuned to the high frequency end of the tunable band by tuning the individual cavities, the Qs of the individual cavities have to be readjusted to obtain the same electronic bandwidth at the high frequency end of the tunable band of the tube. Thus, it is desired to obtain a method for loading the cavities of the driver section of the amplifier in such a manner that a separate adjustment of the cavity Qs, upon tuning of the tube, is avoided.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of improved apparatus for loading cavity resonators of a tunable velocity modulation tube.

In one feature of the present invention, at least one of the driver cavities of the driver section of a velocity modulation tube is loaded by means of a resonant load with the frequency of the resonant load selected relative to the tunable band of the tube such that the resonant load introduces a frequency dependent amount of loss into the loaded cavity resonator for varying the Q of the resonator over the tunable operating range of the tube to substantially maintain the electronic bandwidth of the tube over the tunable range of the tube, whereby separate adjustment of the cavity loading is avoided with tuning of the tube over the tunable band.

In another feature of the present invention, the resonant cavity loading means comprises a series resonant load disposed within an evacuated portion of the loaded cavity resonator, whereby fabrication of the loaded cavity is facilitated and the resultant loaded cavity structure simplified.

In another feature of the present invention, the series resonant loading circuit for loading the cavity resonators comprises a capacitive member disposed adjacent an inside wall of the cavity and supported from the inside wall of the cavity via the intermediary of an inductive support arm, and wherein the resistive loss material is incorporated into the series resonant circuit by being coated onto the outside surface of the inductive support arm.

In another feature of the present invention, the series resonant cavity loading circuit includes a conductive ring having a gap therein to define a capacitor in series with the inductance of the ring, and wherein the ring is coated with a wave attenuative material to introduce resistive loss into the series resonant circuit.

In another feature of the present invention, the series resonant loading circuit includes a capacitive plate sup ported adjacent an inside wall of the cavity via the intermediary of an inductive support arm which is coated with a resistive material, and wherein the support arm is made of a sandwich construction of a plurality of metallic members sandwiched together, and wherein one of the sandwiched metallic members has a thermal conductivity substantially greater than that of a second one of the sandwiched members and wherein the second one of the sandwiched members is made of a material having a strength substantially above that of the first member.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinal sectional view of a klystron amplifier tube incorporating features of the present invention,

FIG. 2 is a plot of power output vs. frequency deviation depicting the electronic bandwidth, tuning and Qs of the cavities of the tube of FIG. 1,

FIG. 3 is a plot of power output vs. frequency deviation depicting the electronic bandwidth of the tube of FIG. 1 and 2 when tuned to the high frequency end of the tunable band of frequencies, and depicting the deficiencies of the prior art,

FIG. 4 is a plot of power output vs. frequency deviation depicting the electronic bandwidth for a prior art klystron power amplifier set for the high frequency end of the tunable band but tuned to the low frequency end of the tunable band,

FIG. 5 is a plot of power output vs. frequency deviation depicting the electronic bandwidth of a prior art tube tuned for proper performance at the high frequency end of the tunable band,

FIG. 6 is a schematic equivalent circuit diagram for a floating resonator loaded by means of a series resonant loading circuit of the present invention,

FIG. 7 is a plot of cavity loaded 0,, vs. frequency depicting a family of curves for a cavity loaded by a series resonant loading circuit of FIG. 6 for various ratios of inductance and capacitance,

FIG. 8 is a schematic longitudinal sectional view of a floating cavity resonator incorporating an internal series resonant loading circuit of the present invention,

FIG. 9 is a sectional view of the structure of FIG. 8 taken along line 99 in the direction of the arrows and rotated 90 for clarity,

FIG. is a sectional view of a portion of the structure of FIG. 9 taken along line 10-10 in the direction of the arrows,

FIG. 11 is a schematic longitudinal sectional view of a floating cavity resonator loadedwith a series resonant loading circuit of the present invention, and

FIG. 12 is a sectional view of the structure of FIG. 1 1 taken along line 12-12 in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown a velocity modulation amplifier tube 1 incorporating features of the present invention. The velocity modulation amplifier tube 1 includes an electron gun 2 disposed at one end of the tube for forming and projecting a beam of electrons 3 over an elongated beam path to a beam collector structure 4 disposed at the terminal end of the beam for collecting and dissipating the energy of the beam.

A klystron driver section 5 is disposed along the beam path in electromagnetic energy exchanging relation therewith for velocity modulating the beam with signal energy to produce a subsequent current density modulation of the beam downstream of the driver section 5. An output circuit 6, such as a re-entrant cavity resonator, is disposed downstream of the driver section 5 in electromagnetic energy exchanging relation with the beam for extracting the amplifier signal wave energy from the beam and for coupling such energy to a suitable utilization device or load, such as an antenna, via a coupling loop and coaxial line 7.

The klystron driver section 5 includes an input circuit 8, such as a re-entrant cavity resonator, excited with signal wave energy via an input coaxial line and coupling loop 9. Input signal wave energy to be amplified is applied to the input resonator 8 for velocity modulating the beam 3 passable therethrough. The velocity modulation is converted into current density modulation in the successive drift tube and excites the succeeding floating resonator 11 in such a phase to produce further velocity modulation and bunching of the beam in the next successive resonator 12, which inturn further velocity modulates the beam to produce current density modulation in the third floating resonator 13. The third resonator 13 in-turn interacts back on the electron beam to produce a tight bunching of the electrons at the output gap of the output circuit 6.

Resonators ll, 12 and 13 are referred to in the art as floating resonators, as they include no sources of radio frequency energy other than the exitation of such resonators via the modulated beam passable therethrough.

Tuners 14 are provided in each of the cavity resonators 8, 1 1-13 and 6 for tuning the electronic bandwidth of the tube over a tunable band of frequencies. More particularly, at the low end of the tunable band of the tube, as depicted in FIG. 2, the individual resonators may be tuned for frequencies as indicated by the arrows upstanding from the abscissa. The arrows are identified with numerals in circles corresponding to the respective position of the resonator arranged in an ascending order taken in the downstream direction.

Thus, from FIG. 2 it is seen that the input resonator 8 is tuned to the center of the electronic band, floating resonator 11 is tuned to the low frequency end of the electronic band, the floating resonator 12 is tuned to the high frequency end of the electronic band, floating resonator 13 is tuned outside the upper end of the electronic band, and the output resonator, is tuned to the carrier frequency f, of the channel which corresponds to a position near the lower end of the electronic band because the tube is to be utilized for single sideband transmission with the upper side band being that which is transmitted.

Thus, when the tube is tuned for carrier frequency of 575 MHZ there is 6 MHz of electronic bandwidth between the carrier frequency f and the upper band edge, and 2 MHz of electronic bandwidth between the carrier frequency and the lower band edge, where the band edges are defined as 1 dB points in the gain and power output characteristic of the tube.

The individual cavity resonators are loaded to have Qs as indicated above the respective arrows. More specifically, resonator 11 is loaded to a loaded Q of 200, the output resonator 6 is loaded to a Q of 36, the input resonator 8 is loaded to a Q of 100, resonator 12 is loaded to a Q of 200 and a resonator 13 is loaded to a Q of 41 I. With this arrangement of loading and tuning the electronic bandwidth of the tube at the low end of the tunable end of the tube is substantially flat over an operable electronic bandwidth of approximately 8 MHz. However, in the prior art, when the tuners 14 are adjusted for tuning the electronic bandwidth to the upper end of the tunable band, as for example, such that the carrier frequency f, is tuned for 704 MHz the Qs of the individual resonators, without subsequent adjustment in the prior art, are no longer proper to obtain optimum electronic bandwidth and therefore the electronic bandwidth of the tube falls off as generally indicated by curve 16 of FIG. 3.

Alternatively, when the tube is tuned to the high end of the band and the Qs are arranged for optimum electronic bandwidth at the high end of the tunable band, as shown by curve 17 of FIG. 5, with subsequent tuning of the tube to the low end of the tunable range, as shown in FIG. 4, the Qs of the cavities, without adjustment in the prior art are no longer proper to obtain uniform gain or power output characteristic over the desired 8 MHz electronic bandwidth. This is indicated by curve 18 of FIG. 4.

For a tube tuned and adjusted for the low end of the tunable range, as shown in FIG. 2, the problem is that the Q's of at least floating resonators 11 and 12, which are tuned to the opposite band edges of the electronic band, do not increase sufficiently to retain the gain near the band edges at the high end of the tunable band. Conversely, if the Qs are increased for the floating resonators 11 and 12 at the high frequency end of the tube, as indicated in FIG. 5, these Qs remain too high when the tube is tuned to the low end of the tunable band, as indicated in FIG. 4.

In the prior art, this change in the Qs for flat electronic bandwidth required at opposite ends of the tunable band was obtained by externally loading the floating resonators and adjusting the coupling between the external loading and the resonators with changes in the carrier frequency of the electronic band to obtain a flat response over the electronic bandwidth.

In the present invention, as shown in FIGS. 6-10, the floating resonators l1 and 12 are each loaded with a series resonant lossy circuit, such circuit preferably being located within the respective floating resonator. The equivalent circuit for the loaded cavity is shown in FIG. 6 where C represents the capacitance of the interaction gap of the cavity resonator, L represents the inductance of the cavity resonator, L represents the inductance of the series resonant loading circuit coupled to the floating resonator via mutual inductance M, C represents the series capacitance of the loading circuit, and R represents the series resistance of the load of the series resonant circuit.

Referring now to FIG. 7, there is shown a family of curves depicting the loaded cavity Q for the floating cavity resonator of FIG. 6 as loaded by the series resonant frequency f, for three capacitance to inductance ratios. More particularly, when the series resonant frequency f, is below the operating tunable range of the tube and of the floating cavity, the Q of the loaded cavity increases with increasing frequency. When the resonant frequency f of the series loading circuit is above the tunable band of the tube and of the floating resonator, the loaded Q decreases with increasing frequency. When the resonant frequency f, of the series loading circuit is tuned within the tunable band of the floating resonator, the loaded Q of the resonator increases over a portion of the band and decreases over another portion of the band. The changes in loading effects are more pronounced with higher ratios of capacitance to inductance.

Referring now to FIG. 8 there is shown a physical realization for the equivalent circuit of FIG. 6. More particularly, a toroidal-shaped re-entrant cavity resonator 11 has a series resonant loading circuit 25 mounted therein. The series resonant loading circuit 25 includes a disc-shaped capacitive plate 26, as of copper, disposed adjacent an end wall 27 of the resonator 11 to define the capacitance C of the series resonant circuit 25. In a typical example for a tube tuned as indicated in FIG. 2, the capacitive plate 26 has a diameter of l'rfiinches and is spaced by 0.195 inch from the adjacent end wall cavity.

The plate 26 is supported from the side wall of the cavity 11 via the intermediary of an arcuate support arm 28 which defines the inductance L of the series resonant loading circuit 25. In a typical example for operation at a frequency of approximately 575 MHz, the support arm 28 is formed of a portion of a ring having an inside radius of 1% inches and an outside radius of inches and is of square cross-section, as shown in FIG. 10. For strength and improved thermal conductivity the support arm 28 is formed of a sandwich construction wherein a relatively high strength stainless steel core member 29 is sandwiched between a pair of thermally conductive outer ring segments 31, as of copper.

The support arm is fixed to the center of the capacitive disc 26 and is supported at the other end from the side wall of the resonator 11. A stainless steel rod 32 is disposed externally of the cavity wall running lengthwise thereof for adding strength to the side wall of the resonator 11 at the point where the support arm 28 is joined to the side wall of the cavity.

A coating of wave attenuative material 33 is provided on the outside of support arm 28 to provide the resistive losses of the series resonant loading circuit 25. In an typical example, the support arm 28 is coated with Kanthal which comprises by weight, 5.5 percent aluminum, 22 percent chromium, 0.5 percent cobalt and the balance iron and marketed under the trade name Kanthal A-l by the Kanthal Corporation of Bethel, Conn. The coating 33 is applied to any suitable thickness as of 0.001 to 0.005 inch.

The base of the support arm 28 is fixedly secured to the side wall of the cavity 11 via the intermediary of a pair of stainless steel screws 34 passing through the stiffening rod 32, the cavity wall 11, and into the end of the support arm 28. In addition, the support arm 28 is brazed to theinside wall of the cavity to form a vacuum-tight seal such that the series resonant circuit 25 is disposed entirely within the evacuated cavity resonator 11.

The face of the capacitive disc 26, which is disposed adjacent the end wall 27 of the resonator l l, is serrated with a multitude of grooves 35 passing diagonally across the face of the capacitive disc 26 for capturing secondary electrons to reduce the possibility of multipactor between the capacitive plate 26 and the opposed wall 27 of the resonator. In addition, the plate 26 is dished inwardly to provide a non-uniform spacing in the capacitive gap to further reduce the possibility of multipactor.

In operation, the series loaded cavity resonator of FIGS. 8-10 has a loaded Q, for its dominant operating mode of oscillation for interaction with the beam, that varies from 200 to 800 over the tunable operating band of the tube which runs from 575 MHz to 704 I-IMz. When the floating resonators 11 and 12 are tuned to opposite band edges, as depicted in FIG. 2, and loaded as indicated, the 8 MHz electronic bandwidth is sustained over the entire tunable range of the klystron amplifier from 575 MHz to 704 MHz without any further adjustment of the Qs of the individual resonators.

Referring now to FIGS. 11 and 12, there is shown an alternative physical realization of the equivalent circuit of FIG. 6. More particularly, the cavity 11 is substantially the same as previously described with regard to FIG. 8 and the series resonant loading circuit 37 comprises an electrically conductive plate 38, as of copper, having a central aperture to define a ring structure. The ring structure forms the inductor of the series resonant loading circuit 37. The ring is slotted at one corner to define a capacitive gap 39, as of 0.025 inch wide. The capacitive gap 39 forms the capacitive element C of the series loading circuit. The resistive loss is inserted into the series resonant loading circuit by coating the surface of the ring at 41 with a lossy wave attenuative material, such as Kanthal, as aforedescribed. The ring is brazed into the cavity such that the plane of the ring projects radially inward toward the capacitive gap C of the toroidal re-entrant cavity.

The advantage of the series resonant loading circuits, as hereinabove described is that such circuits do not require separate adjustment over the tunable band of the tube. Moreover, in the embodiments, as shown in FIGS. 8-12, the series loading circuit is incorporated entirely within the evacuated envelope of the cavity, thereby substantially reducing the complexity and facilitating fabrication of the loaded cavities.

What is claimed is:

1. In a tunable high frequency velocity modulation beam tube, means for projecting a beam of electrons over a predetermined beam path, a plurality of cavity resonator means arranged along said beam path in electromagnetic energy exchanging relation therewith for velocity modulating the electron beam with high frequency signal energy to produce subsequent current density modulation of the beam in accordance with the high frequency signal energy impressed on said beam, means for tuning a plurality of said cavity resonator means over a band of frequencies for changing the operating band of the tube, resonant loading means electromagnetically coupled to at least one of said tunable cavity resonator means for coupling loss into the dominant operating mode of oscillation of said cavity to reduce the Q of the dominant operating mode, said resonant loading means having a substantially fixed resonant frequency related to the operating tunable frequency range of the tube such that said resonant loading means introduces a frequency dependent amount of loss into said loaded cavity resonator means which automatically varies the Q of said loaded cavity over the tunable operating range of frequencies of the tube so as to automatically substantially maintain the electronic bandwidth of the tube over the tunable frequency range of the tube.

2. The apparatus of claim 1 wherein said loaded cavity resonator means is tuned over the tunable frequency range of the tube for a frequency which is substantially at one edge of the electronic bandwidth of the tube, and wherein the resonant frequency of said resonant loading means is set for and remains substantially fixed at a series resonance frequency below the tunable band of the tube, whereby the Q of the loaded cavity resonator means increases with an increase in the tuned operating frequency of the tube.

3. The apparatus of claim 1 wherein said loaded cavity resonator includes at least an evacuated portion and wherein said resonant loading means comprises a lossy series resonant circuit means disposed within said evacuated portion of said loaded cavity resonator means.

4. The apparatus of claim 3 wherein said lossy series resonant circuit means includes an electrically conductive capacitive member disposed adjacent the inside wall of said cavity in spaced relation therefrom to define a capacitor of said series resonant circuit means, an electrically conductive support arm affixed to said capacitive member and extending to an inside wall of said cavity resonator means to form an inductor of said series resonant circuit, and a wave attenuative coating disposed on said support arm to define a resistive loss of said lossy series resonant loading circuit.

5. The apparatus of claim 4 wherein the face of said capacitor member which faces the adjacent wall of said cavity is serrated to inhibit multipactor.

6. The apparatus of claim 4 wherein said support arm includes a sandwich of a plurality of metallic members, a first one of said sandwiched metallic members having a strength substantially above that of at least a second one of said sandwiched members, and said second one of said sandwiched metallic members having a thermal conductivity substantially above that of said first sandwiched member.

7. The apparatus of claim 3 wherein said series resonant loading circuit includes a conductive ring to define the inductance of said series resonant circuit, a gap formed in said ring to define a capacitor in series with said inductor, and wherein said ring is coated with a wave attenuative material to introduce resistive loss into said series resonant circuit.

8. The apparatus of claim 1 including an input circuit means disposed in electromagnetic energy exchanging relation with said beam for coupling high frequency signal energy onto said beam, output circuit means coupled to said beam downstream of said input circuit for extracting amplifier signal wave energy from said beam, and wherein said cavity resonator means, as loaded by said resonant loading means, is a floating cavity resonator disposed along the beam path intermediate said input circuit means and said output circuit means. 

1. In a tunable high frequency velocity modulation beam tube, means for projecting a beam of electrons over a predetermined beam path, a plurality of cavity resonator means arranged along said beam path in electromagnetic energy exchanging relation therewith for velocity modulating the electron beam with high frequency signal energy to produce subsequent current density modulation of the beam in accordance with the high frequency signal energy impressed on said beam, means for tuning a plurality of said cavity resonator means over a band of frequencies for changing the operating band of the tube, resonant loading means electromagnetically coupled to at least one of said tunable cavity resonator means for coupling loss into the dominant operating mode of oscillation of said cavity to reduce the Q of the dominant operating mode, said resonant loading means having a substantially fixed resonant frequency related to the operating tunable frequency range of the tube such that said resonant loading means introduces a frequency dependent amount of loss into said loaded cavity resonator means which automatically varies the Q of said loaded cavity over the tunable operating range of frequencies of the tube so as to automatically substantially maintain the electronic bandwidth of the tube over the tunable frequency range of the tube.
 2. The apparatus of claim 1 wherein said loaded cavity resonator means is tuned over the tunable frequency range of the tube for a frequency which is substantially at one edge of the electronic bandwidth of the tube, and wherein the resonant frequency of said resonant loading means is set for and remains substantially fixed at a series resonance frequency below the tunable band of the tube, whereby the Q of the loaded cavity resonator means increases with an increase in the tuned operating frequency of the tube.
 3. The apparatus of claim 1 wherein said loaded cavity resonator includes at least an evacuated portion and wherein said resonant loading means comprises a lossy series resonant circuit means disposed within said evacuated portion of said loaded cavity resonator means.
 4. The apparatus of claim 3 wherein said lossy series resonant circuit means includes an electrically conductive capacitive member disposed adjacent the inside wall of said cavity in spaced relation therefrom to define a capacitor of said series resonant circuit means, an electrically conductive support arm affixed to said capacitive member and extending to an inside wall of said cavity resonator means to form an inductor of said series resonant circuit, and a wave attenuative coating disposed on said support arm to define a resistive loss of said lossy series resonant loading circuit.
 5. The apparatus of claim 4 wherein the face of said capacitor member which faces the adjacent wall of said cavity is serrated to inhibit multipactor.
 6. The apparatus of claim 4 wherein said support arm includes a sandwich of a plurality of metallic members, a first one of said sandwiched metallic members having a strength substantially above that of at least a second one of said sandwiched members, and said second one of said sandwiched metallic members having a thermal conductivity substantially above that of said first sandwiched member.
 7. The apparatus of claim 3 wherein said series resonant loading circuit includes a conductive ring to define the inductance of said series resonant circuit, a gap formed in said ring to define a capacitor in series with said inductor, and wherein said ring is coated with a wave attenuative material to introduce resistive loss into said series resonant circuit.
 8. The apparatus of claim 1 including an input circuit means disposed in electromagnetic energy exchanging relation with said beam for coupling high frequency signal energy onto said beam, output circuit means coupled to said beam downstream of said input circuit for extracting amplifier signal wave energy from said beam, and wherein said cavity resonator means, as loaded by said resonant loading means, is a floating cavity resonator disposed along the beam path intermediate said input circuit means and said output circuit means. 